1. Field of the Invention
This invention relates to a method of producing aluminum alloy castings and to a piston made of an aluminum alloy.
2. Description of the Prior Art
Generally, diesel engine pistons are made of a high-strength alloy of aluminum containing silicon (e.g., JIS AC8A) and having a small thermal expansion coefficient and high resistance to abrasion. Since the head of the piston is subjected to corrosion from pressurized fuel injected from a fuel injection nozzle, and the ring grooves of the piston are subjected to repeated loading by pressure from the piston rings corresponding to the pressure of the burning air-fuel mixture, it has been strongly desired to improve the high-temperature hardness of these parts, thereby improving resistance to abrasion (resistance to corrosion) and resistance to fatigue.
There has been known a method of improving the surface strength of an aluminum alloy casting in which the aluminum alloy is cast around a metal insert of a different metal by gravity casting or die casting. However, when the metal insert is a porous metal or metallic-fiber molding having continuous pores, molten aluminum alloy cannot penetrate into inner pores in the case of gravity casting, and accordingly, the aluminum alloy cannot be bonded on the surface of the metal insert with sufficient strength. Therefore, debonding and cracking are apt to occur when heat load acts thereon. On the other hand, in the case of die casting, though the bonding strength and packing density are improved, it is very difficult to prevent air from being entrapped in the castings. Since the castings blister due to expansion of the entrapped air when heated, it is difficult to heat-treat the castings in order to improve the strength of the aluminum alloy or to improve the resistance to abrasion and fatigue by forming an intermetallic compound or a solid phase diffusion layer.
In U.S. Pat. No. 4,334,507, there is disclosed another method in which an aluminum alloy is cast around a heat-resistant porous material insert such as a sintered material of Ni-Cr system by high-pressure squeeze casting. In this case, though the aluminum alloy can be charged into the pores of the porous material insert, the bonding force between the porous material insert and the aluminum alloy is still insufficient, and at the same time, the resistance to abrasion and fatigue cannot be sufficiently improved since substantially no intermetallic compound is formed and without the intermetallic compound, the sintered material cannot have sufficient high-temperature hardness.
In Japanese Unexamined Patent Publication No. 54(1979)-151715, there is disclosed another method in which porous metal material such as of nickel is dipped into molten aluminum to close the pores of the metal material on its surface, and is heat-treated to form a layer of a compound of nickel and aluminum on its surface, and around the insert material thus obtained is cast an aluminum alloy. Though the compound layer somewhat contributes to improvement in heat resistance and corrosion resistance in the castings in accordance with this method the bonding strength between the compound layer and the aluminum alloy is insufficient and furthermore, the effect of the compound layer on heat resistance (high-temperature hardness) and corrosion resistance is limited since the compound layer is formed only on the surface of the insert material.
In the art of producing pistons of aluminum alloy, there is a method in which a Ni-resist cast iron insert is alfin-treated and an aluminum alloy is cast around the insert of alfin-treated Ni-resist cast iron so that the insert forms the ring support portion of the piston, a ring groove being machined along the outer periphery of the ring support portion. In the alfin treatment, the Ni-resist cast iron insert is dipped into molten aluminum alloy and then an aluminum alloy is cast therearound with the aim of improving bonding strength between the aluminum alloy and the insert. However, the alfin treatment is troublesome and increases the manufacturing cost, while, at the same time, the bonding strength cannot be sufficiently improved. Furthermore, since when an alfin-treated material is heat-treated, debonding occurs between the alfin-treated layer and the aluminum alloy layer, it is difficult to solution-heat-treat (one kind of heat-treatment) the alfin-treated material in order to improve the mechanical properties of the aluminum alloy layer. Further, since Ni-resist cast iron has a low thermal conductivity, heat transmission from the piston to the cylinder wall is limited to adversely affect cooling of the piston.